HVDC power transmission is becoming increasingly important due to the steadily rising need for bulk power delivery and interconnected power transmission and distribution systems.
An HVDC grid typically comprises multiple alternating current (AC)/DC converter terminals interconnected by transmission lines, i.e., underground cables and/or overhead lines. Within the grid, a terminal may be connected to multiple terminals resulting in different types of topologies. Such a multiple terminal grid enables efficient congestion management and has an improved stability against disturbances.
DC circuit breakers are commonly used for isolating faulty components, such as transmission lines, in HVDC grids. Due to the low inductance of DC transmission lines, as compared to AC systems, HVDC systems suffer from a high rate of rise of fault induced currents. Thus, the DC breakers have to interrupt the current quickly, before the interrupting capability of the breakers is exceeded. In particular, in the event of a breaker failure, i.e., if a circuit breaker fails to interrupt the current, a backup circuit breaker has to be tripped before the rising current exceeds the interrupting capacity of the backup breaker. Thus, unless very large inductances are used for limiting the rate of increase of fault induced currents, a fast detection of DC circuit breaker failures is desirable.
Known breaker failure detection techniques are based on measuring the total current through the DC circuit breaker, and checking whether the total current decreases to zero within a given time interval, starting after the reception of a trip signal. However, the decay of the current is dictated by the non-linear resistor, e.g., a surge arrester, which is typically employed for extinguishing the current, resulting in rather long time intervals. Furthermore, since the current decay is dependent on various external parameters, such as the load prior to the fault, the location of the fault, and the total inductance of the involved circuits, a safety margin has to be taken into account, resulting in even longer time intervals.